1. Field of the Invention
The present invention relates to a process for continuously producing dimethyl carbonate. More particularly, the present invention relates to an industrial process for continuously producing dimethyl carbonate by a catalytical reaction in gas phase of carbon monoxide with methyl nitrite in the presence of a solid catalyst, while effectively and safely recovering methyl nitrite and nitrogen monoxide from a purge gas withdrawn from a gas system circulating through a reaction system, and recycling the recovered methyl nitrite and nitrogen monoxide to the production of dimethyl carbonate.
Dimethyl carbonate is a compound useful as a material for synthesizing aromatic polycarbonates, medicines and agricultural chemicals, and as a solvent.
2. Description of the Related Art
A conventional industrial process for producing dimethyl carbonate by a catalytical reaction in gas phase of carbon monoxide with methyl nitrite in the presence of a solid catalyst comprises, as disclosed in U.S. Pat. No. 5,214,185, a first step of catalytically reacting carbon monoxide with methyl nitrite in gas phase in the presence of a solid catalyst in a reactor to produce dimethyl carbonate; a second step of absorbing dimethyl carbonate produced in the first step by an absorbing medium consisting of dimethyl oxalate in a dimethyl carbonate-absorbing column (absorbing column), to provide a liquid fraction comprising dimethyl carbonate absorbed by dimethyl oxalate and a non-condensed gas fraction containing nitrogen monoxide; a third step of regenerating methyl nitrite by bringing nitrogen monoxide in the non-condensed gas fraction into contact with molecular oxygen and methyl alcohol in a methyl nitrite-regenerating column (regenerating column); and a fourth step of distil-collecting dimethyl carbonate from the liquid fraction produced in the second step and containing dimethyl oxalate in which dimethyl carbonate is absorbed, in an extract-distilling column and a dimethyl carbonate-distilling column.
In the above-mentioned process, a gas containing carbon monoxide and methyl nitrite circulate through the first step, the second step and the third step. This circulating gas further contains a carbon dioxide gas which is produced as a by-product of the gas phase catalytical reaction of the first step, and an inert gas, for example, nitrogen gas, which is introduced into the circulating gas by accompanying with a NOx gas which is fed into the regenerating column to synthesize methyl nitrite. Therefore, to avoid the accumulation of the above-mentioned gases into high concentrations in the circulating gas, a portion of the circulating gas is continuously purged, as a purge gas, from the gas-circulating system.
The purge gas contains high contents of methyl nitrite and nitrogen monoxide, therefore, methyl nitrite and nitrogen monoxide are recovered from the purge gas.
For the purpose of recovery, for example, U.S. Pat. No. 4,879,401, discloses a recovering method in which a purge gas collected from a methyl nitrite-regenerating step is brought into contact with a molecular oxygen-containing gas and methyl alcohol to regenerate methyl nitrite from nitrogen monoxide contained in the purge gas and absorb the regenerated methyl nitrite together with methyl nitrite contained in the purge gas by methyl alcohol; and the resultant methyl alcohol solution containing methyl nitrite is recycled to the methyl nitrite-regenerating step.
In the above-mentioned third step, the regeneration of methyl nitrite from nitrogen monoxide is applied to only a portion of nitrogen monoxide contained in the circulating gas. However, in the above-mentioned recovering method, to recover and convert almost all nitrogen monoxide in the purge gas to methyl nitrite, the molecular oxygen-containing gas must be fed in a stoichiometric amount or more with respect to the amount of nitrogen monoxide, to the methyl nitrite-recovering step. This addition of the molecular oxygen-containing gas in an excessive amount causes, the resultant circulating gas to contain non-reacted molecular oxygen. Namely, the purge gas containing a high content of methyl nitrite is caused to further contain molecular oxygen gas. This purge gas has an increased risk of explosion. This problem must be solved. Also, since the methyl nitrile is regenerated from nitrogen monoxide in the purge gas which already has a high content of methyl nitrite, the total content of methyl nitrite in the purge gas increases to a level at which methyl nitrile in the purge gas cannot be fully absorbed by methyl alcohol. This problem, also, must be solved.